1. Field of the Invention
This invention relates to a structure floating on a body of water. More particularly, the invention relates to a floating structure from which drilling or production operations are carried out. In its more specific aspects, the invention concerns a floating structure having buoyancy means placed especially with respect to the trough of a design wave so as to minimize mooring forces imposed on the vertical elongated members which anchor the structure, such as those forces which may be caused by passing waves.
2. Setting of the Invention
In recent years there has been considerable attention attracted to the drilling and production of wells located in water. Wells may be drilled in the ocean floor from either fixed platforms in relatively shallow water or from floating structures or vessels in deeper water. The most common means of anchoring fixed platforms include the driving or otherwise anchoring of long piles in the ocean floor. Such piles extend above the surface of the water with a support or platform attached to the top of the piles. This works fairly well in shallower water, but as the water gets deeper, the problems of design and accompanying costs become prohibitive. In deeper water it is common practice to drill from a floating structure.
In recent years there has been some attention directed toward many different kinds of floating structures, for the most part maintained on station by conventional spread catenary mooring lines, or by propulsion thruster units. One scheme recently receiving attention for mooring is employed in the so-called vertically moored platform. One such platform is described in U.S. Pat. No. 3,154,039, issued Oct. 27, 1964. A key feature of the disclosure in the patent is that the floating platform is connected to an anchor base only by elongated parallel members. The members there are held in tension by excess buoyancy of the platform. This feature offers a remedy for one of the major problems arising in the conduct of drilling, or like operations from a floating structure. This major problem is that ordinary hull-type barges or vessels, in response to ocean waves, may exhibit substantial amounts of vertical heave and angular roll motion. Such motions significantly hinder drilling operations. Motion difficulties are alleviated to a degree by use of the so-called semisubmersible vessels or structures in which flotation buoyancy is provided by long, slender vertical bottles or tanks. This design suffers the inconvenience that, if carried to the logical extreme of having very little waterplane area, the unit would become statically unstable, requiring careful reballasting to offset changes in vertical loads, such as drilling hook load (e.g., when pulling drill pipe, etc.) or changes in weight of supplies. Some of those problems are eliminated or at least reduced in the vertically moored platform. Being subjected to tension, the elongated parallel members of the vertically moored platform are substantially inextensible and therefore restrain the platform to move primarily in the horizontal direction. This virtually eliminates heave and roll motions. In vertically moored structures heretofore considered, exceptionally strong mooring would be required to resist the vertical forces which might be imposed upon a structure by the orbital motion of passing waves. The present invention describes a means to minimize the mooring forces imposed by the structure on the elongated members, such as those caused by passing waves.